E-books all look and feel the same, plus they lack texture, smell and make no sound when a page is turned. They're an abstraction of the reading experience.
Everything about handling a book stimulates the senses. Books have weight, texture, colors, smells, and they make noise when pages are turned. Because they vary so wildly in size, typography and paper composition, there's an enormous variety of sensations to be had.
E-readers are great and convenient for grown-ups, because all we want is the content, but young children are at a time of their lives when they need to physically experience the world around them. Physical books are better than e-readers on that count.